The invention relates to the field of electronic reproduction technology and in particular to the direct exposure of printing forms from a digital database, which can be carried out either in an exposer, preferably a flat bed exposer, or directly on a drum of a printing machine. The invention relates in particular to a method and an apparatus for exposing photosensitive material, in particular for exposing a printing form, in which a surface to be exposed of the photosensitive material is scanned by at least one light beam, which is emitted by a light source and, before striking the photosensitive material, is optionally modulated, interrupted and/or deflected, and in which an exposure head is moved at a constant distance from the surface in relation to the latter and to the light source, in order to scan the surface of the photosensitive material with the light beam.
In order to expose printing forms of printing machines, in recent years, in addition to the conventional method of exposure by use of films, exposure methods operating digitally have been used to an increased extent. In the case of these exposure methods, the image information to be transferred to the printing form is supplied in the form of digital signals to an exposure unit. There, the signals are used to drive light sources, in order to switch the light sources on and off or to modulate the intensity of the light beams emitted or in order to interrupt, deflect or modulate the intensity of the light beams emitted by the light sources after they have emerged from the light sources, so that the exposure of each image point or pixel on the printing form corresponds to the image information to be transferred to this point. The scanning of the printing form surface with the light beams is usually carried out by a writing or exposure head, as it is known, which is moved along the surface at a constant distance from the latter and, in the process, deflects the light beams guided to the printing form onto the surface line by line or column by column.
Here, the exposure head may carry the light sources and devices for interrupting, deflecting or modulating the light beams, such as in the case of exposure devices from BasysPrint which, in addition to a UV light source, contain a component that is irradiated with incoherent light from the UV light source and contains a large number of micromirrors. Each of the micromirrors corresponds to one pixel or image point of a raster image produced by the scanning on the printing form and can be driven digitally, in order either to project the incident UV light through an optical system onto the printing form or to deflect the light to such an extent that it is not acquired by the optical system. In addition to the UV light source and the micromirror array, the exposure head of the exposure devices needs a power supply and cooling devices, as a result of which it becomes relatively heavy. In order to move such an exposure head in a meandering fashion over the surface of the printing form, high acceleration forces are needed for acceleration and braking at the start and end of each line, and a great deal of power is consumed.
Furthermore, it is known, for example from U.S. Pat. No. 5,351,617, in a system for exposing printing plates by using digitally controlled laser beams, for the laser beams generated in a plurality of stationary laser light sources and used for the exposure to be led via flexible optical conductors to a moveable exposure head, which is moved along at a constant distance on a flat or cylindrical printing form surface and scans the latter point by point or line by line with laser beams emerging from the optical conductors. However, the exposure head of the system must be equipped between the fiber exit from each optical conductor and the surface of the printing form with a lens system for focusing the laser beams, which, together with the optical conductors and the devices required to couple the laser beams into the optical conductors, contribute not insignificantly to the costs of such a system.
It is accordingly an object of the invention to provide a method and an apparatus for exposing printing forms which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, in which an exposure head is configured to be as light as possible, and without the use of optical conductors.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of exposing a photosensitive material. The method includes the steps of providing a light source emitting a light beam substantially parallel to a surface to be exposed, providing an exposure head and the light beam is deflected through a gaseous medium toward the exposure head and reflected by the exposure head onto the surface to be exposed, and using the light beam to scan the surface of the photosensitive material. The light beam, before striking the photosensitive material, is selectively modulated, interrupted and/or deflected. During the scanning, the exposure head is moved, at a constant distance from the surface, in relationship to the surface and to the light source to scan the surface of the photosensitive material with the light beam.
According to the invention, the object is achieved in that after emerging from the light source substantially parallel to the surface of the photosensitive material, the light beam is deflected through a gaseous medium toward the exposure head and there is reflected onto the surface.
The invention is based on the idea of bridging the varying distance between the moveable exposure head and the light source by use of a light beam guided rectilinearly through the surrounding air, the light beam then being deflected toward the surface of the photosensitive material by a reflector disposed on the exposure head. The parallel alignment of the surface of the photosensitive material and of the light beam before the latter strikes the exposure head ensures that the light beam always strikes the same point on the reflector, irrespective of the position of the exposure head, and therefore when a pulsed light source is used or a plurality of parallel light beams, the distance between two adjacent image points is always constant. The reflector required to deflect the light beam onto the exposure head can be produced with a very low weight, so that high acceleration of the exposure head with low expenditure of force and power is made possible.
According to a preferred refinement of the invention, the alignment of the light beam before it strikes the exposure head preferably corresponds to the direction of the relative movement between the latter and the light source, so that it can be maintained unchanged during the entire exposure time. In the process, the exposure head and the light source are expediently moved alternately toward each other and away from each other in this direction, while the surface of the photosensitive material is scanned line by line by the light beam. In order to scan its surface, the photosensitive material is preferably moved in relation to the exposure head in a direction that is perpendicular to the direction of the relative movement between the exposure head and the light source.
A further preferred refinement of the invention provides for the light source to be formed by a laser diode belonging to a line of laser diodes. The line of laser diodes emits a plurality of parallel laser beams which, after passing through an optical system in order to collimate the laser beams and/or in order to reduce the beam diameter and the spacing of the laser beams parallel to the surface of the photosensitive material, are deflected toward the exposure head through the air between the optical system and the exposure head and there are reflected toward the surface in a parallel alignment.
The method according to the invention is preferably used for the exposure of printing forms with laser light in a flat bed exposer or on a drum of a printing machine. In the first-named case, the exposure head is expediently moved to and fro rectilinearly over the surface of the printing form, while it is supplied with a plurality of parallel laser beams from a plurality of laser diodes. The laser diodes are expediently disposed to be stationary on the other side of one circumferential edge of the printing form, while the printing form, together with a support table for the exposer is displaced under the exposure head in a direction perpendicular to the direction of movement of the exposure head.
In the last-named case, the printing form clamped onto the surface of the drum, together with the drum, is set rotating, while the exposure head is moved in the axial direction along the drum and the printing form in order to scan the printing form surface. In this case, a plurality of parallel laser beams parallel to the axis of rotation of the drum from the light sources disposed on the other side of one end of the printing form is deflected toward the exposure head, and the surface of the printing form is scanned, preferably in the drum circumferential direction. The rotational speed of the drum is expediently chosen such that, during one revolution of the drum, the exposure head is displaced axially by the width of one or more lines running in the circumferential direction.
The movement of the printing form in relation to the exposure head and to the light sources, which in the case of the flat bed exposer is carried out at right angles to the to and fro movement of the exposure head or, respectively, the movement of the exposure head along the drum surface, can be carried out step by step in steps of one or more line widths or continuously, where in the last-named case, the information transmitted by a laser beam must correspond to the desired information on a meandering or helical line inclined with respect to the direction of the movement.
In accordance with an added mode of the invention, there is the step of moving the photosensitive material and the light source is kept stationary.
In accordance with an additional mode of the invention, there is the step of clamping the photosensitive material onto a plane and the exposure head is moved in a meandering fashion over the plane.
In accordance with another mode of the invention, there is the step of clamping the photosensitive material on a drum and the light beam is deflected toward the exposure head in an axial direction of the drum.
In accordance with a further mode of the invention, there is the step of scanning the surface of the photosensitive material by a plurality of light beams. The light beams are deflected parallel to one another through the gaseous medium toward the exposure head and are provided by a plurality of light sources.
In accordance with a concomitant mode of the invention, there is the step of reducing a spacing and a diameter of the light beams between the light sources and the exposure head.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for exposing a photosensitive material. The apparatus contains devices used for scanning a surface to be exposed of the photosensitive material with at least one light beam. The devices include a light source emitting the light beam, and a unit for selective modulation, interruption and/or deflection of the light beam. The unit is disposed downstream of the light source. An exposure head is provided and is movable at a constant distance from the surface in relation to the surface and to the light source for scanning the surface of the photosensitive material with the light beam. The light beam is received by the exposure head, and over at least one part of a distance between the light source and the exposure head, the light beam passes through a gaseous medium substantially parallel to the surface to be exposed. The exposure head contains a reflector for reflecting the light beam onto the surface, and the exposure head is disposed downstream of the unit.
In accordance with an added feature of the invention, a flat support table is provided for carrying the photosensitive material, and the exposure head can be moved in a meandering fashion over the support table.
In accordance with an additional feature of the invention, a drum is provided and the photosensitive material is clamped on the drum. Along the drum, the exposure head is moved in an axial direction and the photosensitive material can be acted on by the light beam in the axial direction.
In accordance with another feature of the invention, the light source is one of a plurality of light sources producing a plurality of parallel light beams for a simultaneous scanning of the photosensitive material. An optical system is disposed between the plurality of light sources and the exposure head. The optical system reduces a spacing and a diameter of the parallel light beams between the light sources and the exposure head. The optical system images the parallel light beams from the plurality of light sources telecentrically on the photosensitive material.
In accordance with a concomitant feature of the invention, the unit is one of a plurality of units for selective modulation, interruption and/or deflection of the light beam. The units produce a plurality of parallel light beams for a simultaneous scanning of the photosensitive material. An optical system is disposed between the units and the exposure head. The optical system images the parallel light beams from the units telecentrically on the photosensitive material.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and an apparatus for exposing printing forms, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.